1. Field of the Invention
The present invention relates to a method of manufacturing low alloy and ultra-low-carbon a cold rolled steel sheet that exhibits excellent resistance to cold-work embrittlement and a small planar anisotropy by the continuous annealing method which is suitable as a pressed steel sheet for use in automobiles.
2. Description of the Related Art
When a cold rolled steel sheet is manufactured, a continuous annealing, including heating and cooling which last for a short period of time, is generally conducted subsequent to the cold rolling. In this continuous annealing process, the material quality of the product is greatly affected by the chemical composition of the material. Hence, to obtain a steel sheet exhibiting excellent deep drawing property and stretchability, it has been the practice to add a carbide/nitride producing component, such as Ti or Nb, to the extra low carbon steel.
However, the steel sheet in which Ti or Nb is present is characterized in that Ti is readily combined with C, S, N or O in the steel to form a precipitate. Consequently, the grain boundary is cleaned and the grain boundary strength is thus greatly reduced, increasing the possibility that a brittle fracture (the fracture due to cold-work embrittlement) will occur after deep drawing. Also, it has been a practice to obtain a high-strength steel sheet by adding Mn, Si or P to the steel material. In that case, however, since Si and P readily embrittle the steel sheet, the resistance to cold-work embrittlement greatly deteriorates. To improve such a drawback, B has been added to the steel in the form of a solid solution to increase the grain boundary strength, like C.
However, it is well known that adding B deteriorates the formability. Therefore, the proportion of B to be added is restricted to such a small value that sufficient resistance to cold-work embrittlement cannot be obtained.
Various other methods of improving the deep drawing property and stretchability of the steel sheet by controlling the conditions of hot rolling, cold rolling or annealing during the manufacturing process of the steel sheet have also been suggested. Generally, the hot rolling finishing temperature is set to an Ar.sub.3 transformation point or above from the viewpoint of improving the deep drawing property. The coiling temperature is between 650.degree. and 800.degree. C. from the viewpoint of improving the formability, especially deep drawing properties. The annealing temperature is set to a relatively low temperature which is equal to or higher than the recrystallization temperature and which is effective in terms of the energy.
Japanese Patent Laid-Open No. 62-278232 discloses a method of manufacturing a cold rolled steel sheet of the aforementioned type for use in non-aging deep drawing by the direct hot-rolling method. Japanese Patent Laid-Open No. 1-177321 discloses a method of manufacturing a cold rolled steel sheet of the aforementioned type which exhibits an excellent deep drawing property. Japanese Patent Laid-Open No. 2-200730 discloses a method of manufacturing a cold rolled steel sheet of the aforementioned type which exhibits an excellent press formability. In any of these methods, although B is added to improve the resistance to cold-work embrittlement, there is no concrete disclosure to exhibit brittle transition temperature. Also, coiling is performed at a high temperature of 640.degree. C. or above which impairs descaling ability in a pickling process. Therefore, in any of these methods, a sufficient improvement in the resistance to cold-work embrittlement cannot be expected.
Japanese Patent Laid-Open No. 63-241122 discloses a method of manufacturing a continuously galvanized steel sheet for use in a super deep drawing. In this method, the proportion of B contained is 0.0010% or below, which is too small to improve the resistance to cold-work embrittlement.
Japanese Patent Laid-Open No. 62-40318 discloses a method of manufacturing a cold rolled steel sheet exhibiting an excellent deep drawing property. Japanese Patent Laid-Open No. 1-188630 discloses a method of manufacturing a cold rolled steel sheet exhibiting an excellent press formability. However, in any of these methods, there is no concrete description of the resistance to cold-work embrittlement, and annealing is conducted at a temperature ranging between the recrystallization temperature and 800.degree. C. Therefore, a sufficient improvement of the resistance to cold-work embrittlement cannot be expected.
Japanese Patent Laid-Open No. 61-133323 discloses a method of manufacturing a steel sheet exhibiting an excellent formability. Japanese Patent Laid-Open No. 62-205231 discloses a method of manufacturing a high-strength steel sheet. Both of these methods are directed to the manufacture of a slab thinner than a normal one and to alleviation or simplification of the rolling process of steel sheet using such a thin slab. However, in the former method, there is no concrete description on the conditions of the annealing which is conducted subsequent to the cold rolling process. Although there is a concrete disclosure of the resistance to cold-work embrittlement, the effect thereof is insufficient. In the latter method, there is a concrete disclosure of the annealing which is conducted at a temperature of 775.degree. C. or below. However, sufficient improvement in the resistance to cold-work embrittlement cannot be expected under such conditions.
In any of the aforementioned conventional methods, it is thus difficult to readily obtain a cold rolled steel sheet exhibiting an excellent deep drawing property and an excellent resistance to cold-work embrittlement.
Planar anisotropy, known as one of barometers of the press formability, is generally evaluated by .DELTA.r. The closer to zero the planar anisotropy value is, the more uniform characteristics in each direction can be obtained, which is desirable in terms of press formability. Japanese Patent Laid-Open No. 61-64852 discloses a method of improving this planar anisotropy by adding a relative large amount of Nb in an extra low carbon steel. Although this method is effective in improving the planer anisotropy, it deteriorates elongation (El) or r value. No method of improving the resistance to cold-work embrittlement as well as the planar anisotropy has been disclosed.
It is disclosed in "Iron and Steel, '73-S191, KUBODERA, NIHON KOKAN (vol. 59, No. 4, Mar. 1973)" that softening and a increase in rvalue can be achieved by high-temperature coiling in the hot rolling step (lines 16 and 17), FIG. 1 shows a relation between the coiling temperature (.degree. C.) and the rvalue.
It is disclosed in "Iron and Steel, '85-S1361, SAYANAGI, SHIN NIHON STEEL (Vol. 71, No. 13, Sep. 1985)" that as the hot rolling heating temperature decreases, and the coiling temperature increases, the workability improves, and the recrystallization temperature decreases (lines 2 to 3).
In this way, conventionally, the r value and elongation are normally increased by decreasing the hot rolling heating temperature, and increasing the coiling temperature. The concept disclosed in U.S. Pat. No. 4,504,326, Tokunaga, utilizes
However, the present invention employs the reverse of the conventional method in which the r value and elongation are increased by increasing the hot rolling heating temperature and decreasing the coiling temperature.